1. Field of the Invention
The present invention relates to apparatuses and methods for the detection of electrical corona discharge and, in particular, to detection of corona discharge under outdoor daylight conditions.
2. Description of the Related Art
High-voltage electrical apparatus often are surrounded by a corona discharge which occurs when the electric field in the surrounding air exceeds the threshold for dielectric breakdown of air. When this occurs, the air surrounding the high-voltage apparatus ceases to be an insulator and becomes partially conducting. For example, apparatus such as electrical power transmission lines, transformer and substation insulators and bushings, high-voltage power supplies, and the like often have coronas associated therewith. It is desirable to detect the position and extent of such coronas in order to detect and address potential equipment failure. These coronas, which are also sometimes referred to as corona discharges, will be referred to herein simply as coronas.
Coronas are typically most easily visible or detectable by various techniques in dim light or darkness. However, there is a need to be able to detect coronas under conditions of outdoor daylight (sunlight).
One technique used to detect and identify the general position of coronas involves the use of ultrasonic microphones. However, ultrasonic microphones do not provide an image and thus cannot precisely locate the source of most corona discharges.
Conventional night-vision equipment which incorporates image intensifier tubes is also sometimes used to locate corona discharges, and can be used to provide an image of a corona, in addition to detecting the corona, unlike ultrasonic techniques. Unfortunately, conventional night-vision equipment has relatively poor sensitivity to the optical energy emitted by a corona discharge, and much better sensitivity to both sunlight and artificial lighting, requiring the equipment to be operated in virtually complete darkness and giving poor sensitivity to corona. This is inconvenient and expensive.
The primary obstacle to practical corona discharge imaging under outdoor daylight conditions is the intense background light from the sun, which is far more intense than the light emitted by corona discharge. Radiation emitted by the sun exhibits a substantially continuous spectrum extending from the far ultraviolet (UV) to the far infrared; this spectrum is roughly equivalent to a "black body radiator" at a temperature of 6,000 degrees Kelvin.
Corona discharges in air associated with high-voltage electrical apparatus emit optical radiation of a mainly non-continuous spectrum; this radiation is concentrated in a large number of narrow spectral bands or "lines" that occur at wavelengths ranging from the far UV to the infrared (the UV spectrum ranges in wavelength from about 200 to 400 nanometers (nm)). The most intense of these lines fall within the UV spectrum at a wavelength range of approximately 295 to 400 nm, a portion of the optical spectrum where solar background radiation is sufficiently intense to interfere with corona discharge imaging under outdoor daylight conditions. These emission lines are primarily a subset of the molecular nitrogen emission spectrum second positive emission lines.
Co-owned U.S. Pat. No. 5,886,344 (the '344 patent) teaches a corona detector which uses narrow-band optical filtering centered at one or more of these narrow emission lines, combined with a suitable UV-sensitive detector, to enable images of corona discharge to be obtained even in the presence of moderate background light. However, in order to obtain corona images outdoors during daylight hours, the corona detector of the '344 patent would require the use of optical filters having an extremely narrow bandwidth (i.e., less than 0.1 nm) in order to provide sufficient attenuation of solar background radiation. Fabrication of optical filters with such an extremely narrow bandwidth in the UV portion of optical the spectrum is impractical using currently-available optical technology, and is likely to remain so in the near future.
There is, therefore, a need for improved outdoor daylight corona detection techniques.